Kenneth Marshall wrote:
I understand that a hash value is a many-to-one mapping. That is the
point of the flag in the index. The flag means that there is only one
item in the heap corresponding to that hash value. In this case we
know that the value in the heap is the correct one and a possibly
very expensive string comparison can be skipped. Given that the hash
function is doing its job, almost every string comparison can be skipped.
How long would it take to compare 1-32K of data? How much CPU usage?
With this field in place, you only need to check tuple visibility
The value comparison cannot be skipped. I do not think you understand the many-to-one mapping in its entirety.


   Table has: a(1), b(2), c(3)
   Index has: 1 => 1 (unique), 2 => 2 (unique), 3 => 3 (unique)


   select * from table where key = 'z';

If 'z' hashes to '3' (completely possible), then the index record 3 points to tuple 3, and it "exists". Only, it doesn't because 'a' <> 'z'. You MUST check the value.



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